Photo-detectors made of semiconductor material are known. They are extraordinarily sensitive to infrared radiation with a wavelengths of 800 nm. The sensitivity of these photo-detectors to light decreases rapidly to negligible values as the wavelengths becomes shorter because the reflection losses at the point of entry into the semiconductor and the absorption of the radiation in the crystal increase sharply. For example, the depth of penetration of radiation of 800 nm into silicon at room temperature is approximately 10,000 nm. The depth of penetration of violet radiation of 400 nm is approximately 200 nm and the penetration depth of radiation in the ultra-violet range with a wavelength of 200 nm is only 10 nm. (H. F. Wolf, Semiconductors, Wiley-Interscience 1971, ISBN 0-471-95949-9). Furthermore, pairs of charged particles are lost near the surface of the semiconductor due to recombination. These losses through recombination affect mainly those charged particles which are produced by the radiation of shorter wavelengths.
In photo-detectors with semiconductor structures having a P-N junction immediately under the surface these recombination losses are considerably reduced near the surface. T. E. Hansen, in "Silicon Detectors for the UV- and Blue Spectral Regions with Possible Use as Particle Detectors", Nuclear Instruments and Methods in Physics Research Vol. A235 (1985) p. 249, describes photo-detectors made of N-type or P-type silicon which are also sensitive to radiations of the ultraviolet spectral region.
The photo-detectors made of P-type silicon have an MOS structure. The MOS structure naturally forms a very thin N.sup.+ inversion layer, at the interface between the insulator and the P-type semiconductor.
The photo-detectors made of N-type silicon in the form of a diode have a thin P.sup.+ type layer doped with boron by ion bombardment at the point of entry of the radiation directly at the surface, said layer, together with the substrate, forming a P-N junction.
Instead of the P-N junction, a Schottky contact to the N-type silicon can also be used.
A PIN diode used as a photodiode for infrared radiation is known to have an intermediate zone located between a P-type and an N-type zone. The intermediate zone is intrinsic and is typically 50 Mm wide.
The reflection losses can be reduced by means of a coating, the thickness of which is adapted to the wavelengths of the light, said coating constituting an entrance window with a lower refraction index than that of the semiconductor material (T. Ando & C-K Fong "Photoresponses in In.sub.2 O.sub.3 -Transparent Gate MOS Capacitors", IEEE Transactions on Electron Devices, Vol. ED-29, No. 8, August 1982, p. 1161).
Ultraviolet-sensitive semiconductor photo-detectors according to the state of the art are still more sensitive by several factors to red light with longer wavelengths than to light with shorter wavelengths of less than 400 nm (W. Schmidt and O. Feustel, Optoelektronik, Vogelverlag, Wu1975, page 62, ISBN 3-8023-0076-9).
These photo-detectors are not suited for applications such as in an instrument to monitor a flame, for example. A selective, and therefore expensive, filter is required to keep light with longer wavelengths away from the detector. Interference filters are known from "Concepts of Classical Optics" by John Strong, 1958, ISBN 0 7167 0301 7.
It is an object of the instant invention to create a photo-detector made of semiconductor material which has a greater sensitivity in the range of wavelengths from 200 nm to 400 nm than in the spectral range above 400 nm.